1. Field of the Invention
The invention relates to acceleration request determining system, acceleration request determining method, and control system and control method of an internal combustion engine. In particular, the invention relates to acceleration request determining system and method and control system and method of an internal combustion engine, which improve the output performance of the engine and the fuel economy when appropriate, while assuring sufficient levels of the output performance and the fuel economy at the same time.
2. Description of the Related Art
As a control technology for an internal combustion engine, it is known to determine whether a request for acceleration is made in order to, for example, improve the output performance of the engine. For example, the presence of a request for acceleration is determined based on, for example, the opening of a throttle valve that is controlled in accordance with the amount of depression of the accelerator pedal, as described in the Japanese patent application publication No. JP-A-2004-245104 as one embodiment of the invention. Also, in an internal combustion engine having a turbocharger and a variable valve actuating mechanism, it is known to extend a valve overlap, namely, a period in which intake valves and exhaust valves are both open. For example, the Japanese patent application publication No. JP-A-2004-245104 describes a turbo charged engine in which the valve overlap of the intake and exhaust valves is extended when a request for acceleration is made. Also, the Japanese patent application publication No. JP-A-H11-257109 describes an air-fuel ratio control system of an internal combustion engine, which extends the valve overlap when the engine operates at a rich air-fuel ratio, such as when the engine operates at a high load. In the turbo charged engine of the Japanese patent application publication No. JP-A-2004-245104, the valve overlap is extended so as to bring about secondary combustion of unburned HC, thereby to reduce a turbo lag of the turbocharger. The air-fuel ratio control system of the Japanese patent application publication No. JP-A-H11-257109 extends the valve overlap so as to make the exhaust air-fuel ratio equal to the stoichiometric ratio or lean, thereby to maintain the converting or purifying capability of a catalyst at a sufficiently high level and thus prevent an increase of unburned HC in the exhaust gas.
In the meantime, the pressure measured at the upstream side of the throttle valve (which will be simply referred to as “upstream pressure”) changes with changes in the environment and also changes with time. More specifically, in the case where the vehicle is at a high altitude, for example, the atmospheric pressure is low; therefore, the intake air density is reduced, and the upstream pressure is also reduced. In a vehicle equipped with a turbo charged engine, for example, if the cooling efficiency of an intercooler is reduced, the intake air density decreases as the ability to cool the intake air deteriorates, and a pressure loss of intake air in the intercooler increases as the intake temperature increases. As a result, the upstream pressure of the throttle valve located downstream of the intercooler is also reduced.
FIG. 7A and FIG. 7B schematically illustrate problems encountered in an internal combustion engine in the case where the presence of a request for acceleration is determined based on the accelerator pedal position or the opening of the throttle valve. More specifically, FIG. 7A shows the relationships between the opening of the throttle valve and the intake air amount (or flow rate of intake air), with respect to the case where the vehicle is in normal conditions and the case where the vehicle is, for example, at a high altitude. FIG. 7B shows the relationships between the opening of the throttle valve and the pressure measured at the downstream side of the throttle valve (which will be simply called “downstream pressure”), with respect to the case where the vehicle is in normal conditions and the case where the vehicle is, for example, at a high altitude. In FIG. 7A and FIG. 7B, the vehicle is said to be, for example, at a high altitude when it is subjected to any environmental changes or chronological changes resulting in a reduction of the upstream pressure of the throttle valve, and the vehicle is said to be in normal conditions when subjected to none of such changes. In FIG. 7A and FIG. 7B, the horizontal axis that indicates the opening of the throttle valve has the same scale.
As shown in FIG. 7A, when the vehicle is, for example, at a high altitude, the intake air amount is reduced as compared with the normal case (i.e., the case where the vehicle is in normal conditions) with respect to the same opening of the throttle valve. In this case, it is necessary in the case where the vehicle is, for example, at a high altitude to increase the opening of the throttle valve so as to provide the same amount of engine output or power as that provided in the normal case. Since the fuel injection control is generally performed based on the throttle opening and the engine speed, the air fuel ratio goes out of an appropriate range at this stage. This condition may also be recognized in FIG. 7B, in which the downstream pressure gradually increases in both of the cases as the opening of the throttle valve increases, whereas the upstream pressure is lower in the case of a high altitude than that in the normal case. Thus, in the normal case, the upstream pressure and the downstream pressure become substantially equal to each other when the throttle valve is opened by a sufficiently large degree. In the case where the vehicle is, for example, at a high altitude, on the other hand, the upstream pressure and the downstream pressure become substantially equal to each other at an earlier point in time, namely, before the throttle valve is sufficiently largely opened. In the following description, the condition in which the upstream pressure and the downstream pressure become substantially equal to each other will be simply called WOT (Wide Open Throttle) point. It will be understood from the above description that the WOT point changes if the upstream pressure of the throttle valve changes. Once the throttle opening reaches the WOT point, it cannot be expected to increase the intake air amount by opening the throttle valve by a larger degree than that of the WOT point, and it is thus necessary to improve the intake charging efficiency by use of, for example, a turbocharger, so as to further improve the output performance of the engine.
Suppose that the presence of a request for acceleration is determined when the opening of the throttle valve that is controlled in accordance with the amount of depression of the accelerator pedal reaches opening X2 as indicated in FIG. 7B. In this case, where the vehicle is, for example, at a high altitude, it is not determined that a request for acceleration is made until the accelerator pedal is depressed to such an extent that the throttle opening becomes equal to or larger than the opening X2. Namely, since it cannot be expected to increase the intake air amount by opening the throttle valve by a degree equal to or larger than that of the WOT point, the engine is insensitive to changes in the throttle opening (or the amount of depression of the accelerator pedal) in a region from the throttle opening of the WOT point to the opening X2 in the case where the vehicle is, for example, at a high altitude. Consequently, the output performance of the engine is improved when appropriate so as to match the intention of the driver who depresses the accelerator pedal in an attempt to increase the output or power of the engine, resulting in deterioration of the driveability. Suppose that the presence of a request for determination is determined when the opening of the throttle valve that is controlled in accordance with the amount of depression of the accelerator pedal reaches opening X1 as indicated in FIG. 7B. In this case, when the accelerator pedal is depressed in the normal case, it is determined that a request for acceleration is made at a point in time earlier than the time when the throttle opening reaches the WOT point for the normal case, and, therefore, a process for improving the output performance is performed based on the request for acceleration even if the engine is still capable of generating power by itself. In this case, the output performance is improved while sacrificing the fuel economy (i.e., with a result of reduction of the fuel economy), and the balance between the fuel economy and the output performance may deteriorate. Thus, the method of the related art in which the presence of a request for acceleration is determined based on the throttle opening or accelerator pedal position cannot satisfactorily deal with the situation where the WOT point changes with environmental changes and/or chronological changes. Thus, the method of the related art is not able to favorably improve the output performance and fuel economy of the engine based on a request for acceleration, while assuring sufficient levels of the output performance and fuel economy at the same time.